1. Field of the Invention
The present invention relates generally to intensity modulated radiation therapy (IMRT), and more particularly, to a segmentation algorithmic approach to IMRT.
2. Related Art
Intensity Modulated Radiation Therapy (IMRT) is a new technique for cancer treatments and aims to deliver a highly conformal dose to the target volume while keeping the dose to other surrounding organs at risk (OAR) below tolerance levels, see T. R. Bortfeld, A. L. Boyer, W. Schlegel, D. L. Kahler, and T. L. Waldron, Experimental verification of multileaf conformal radiotherapy, In A. R. Hounsell, J. M. Wilkinson, and P. C. Williams, editors, Proc. 11th Int. Conf. on the Use of Computers in Radiation Therapy, pages 180-181, 1994; T. R. Bortfeld, A. L. Boyer, W. Schlegel, D. L. Kahler, and T. L. Waldron, Realization and verification of three-dimensional conformal radiotherapy with modulated fields, Int. J. Radiat. Oncol. Biol. Phys., 30:899-908, 1994; C. S. Chui, T. LoSasso, and S. Spirou, Dose calculation for photon beam with intensity modulation generated by dynamic jaw or multileaf collimation, Med. Phys., 21:1237-1244, 1994; P. Geis, A. L. Boyer, and N. H. Wells, Use of a multi-leaf collimator as a dynamic missing-tissue compensator, Med. Phys., 23:1199, 1996; C. C. Ling, C. Burman, S. Chui, G. J. Kutcher, S. A. Leibel, T. LoSasso, R. Mohan, T. Bortfeld, L. Reistein, S. Spirou, X. H. Wang, Q. Wu, M. Zelefsky, and Z. Fuks, Conformal radiation treatment of prostate cancer using inverse-planned intensity modulated photon beams produced with dynamic leaf collimation, Int. J. Radiat. Oncol. Biol. Phys., 35:721-730, 1996; X. Wang, S. Spirou, T. LoSasso, J. Stein, C. S. Chui, and R. Mohan, Dosimetric verification of intensity-modulated field, Med. Phys., 23:317-327, 1996; S. Webb, The Physics of Three-Dimensional Radiation Therapy, Bristol, IoP Publishing, 1993; K. R. Winston and W. Lutz, Linear accelerator as a neurosurgical tool for stereotactic radiosurgery, Neurosurgery, 22(3):454-464, 1988; and Cmax. Yu, M. J. Symons, M. N. Du, A. A. Martinez, and J. W. Wong, A method for implementing dynamic intensity modulation using independent jaws and a multileaf collimator, Phys. Med. Biol., 40:769-787, 1995, the entire contents and disclosures of which are hereby incorporated by reference. Implementation of IMRT requires an ability to deliver 2-dimensional non-uniform fluence distributions, called intensity maps or intensity modulated beams (IMBs) (see FIG. 1A).
Currently, IMRT may be delivered by multiple overlapping fields shaped by a multileaf collimator (MLC), (see FIG. 1B for a schematic representation of an MLC), see also T. J. Jordan and P. C. Williams, The design and performance characteristics of a multileaf collimator, Phys. Med. Biol., 39:231-251, 1994, the entire contents and disclosure of which is hereby incorporated by reference. Using a MLC, IMRT may be implemented either statically or dynamically. In dynamic approaches, the MLC leaves keep moving across a treatment field while the radiation remains on. In static approaches, also referred to as “step-and-shoot” IMRT, the MLC leaves stay stationary during irradiation, and move to reshape the beam while the radiation is turned off. A number of dynamic and static MLC techniques have been described previously, see S. Webb, The Physics of Three-Dimensional Radiation Therapy, Bristol, IoP Publishing, 1993; and S. Webb, The Physics of Conformal Radiotherapy, Advances in Technology, Bristol, IoP Publishing, 1997, the entire contents and disclosures of which are hereby incorporated by reference.
Due to the complexity of dynamic intensity modulated beams, the verification of all aspects of the planning and delivery procedures becomes difficult. Thus, “step-and-shoot” is the currently preferred method for delivering IMRT in clinical practices. Advantages of the “step-and-shoot” technique include precise delivery, easy verification, and general availability. A key disadvantage, however, is that it may require a prolonged treatment time, since radiation has to be constantly switched on and off to allow MLC leaves to reshape. In some machines, such as those commercially available from ELEKTA INC., and SIEMENS Corporation, the inter-segment delay dominates the total treatment time in a step-and-shoot delivery. Thus, in such cases, it is highly desirable to use a leaf sequence with the fewest segments possible to deliver the desired intensity map, P. Xia and L. J. Verhey, MLC leaf sequencing algorithm for intensity modulated beams with multiple static segments, Medical Physics, 25:1424-1434, 1998, the entire contents and disclosure of which is hereby incorporated by reference.